net: dsa: microchip: implement PHY loopback configuration for KSZ8794 and KSZ8873

Correct the PHY loopback bit handling in the ksz8_w_phy_bmcr and
ksz8_r_phy_bmcr functions for KSZ8794 and KSZ8873 variants in the ksz8795
driver. Previously, the code erroneously used Bit 7 of port register 0xD
for both chip variants, which is actually for LED configuration. This
update ensures the correct registers and bits are used for the PHY
loopback feature:

- For KSZ8794: Use 0xF / Bit 7.
- For KSZ8873: Use 0xD / Bit 0.

The lack of loopback support was seen on KSZ8873 system by using
"ethtool -t lanX". After this patch, the ethtool selftest will work,
but only if port is not part of a bridge.
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Link: https://lore.kernel.org/r/20240124123314.734815-4-o.rempel@pengutronix.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/microchip/ksz8795.c

@@ -632,6 +632,57 @@ static void ksz8_w_vlan_table(struct ksz_device *dev, u16 vid, u16 vlan)
 	ksz8_w_table(dev, TABLE_VLAN, addr, buf);
 }
 
+/**
+ * ksz879x_get_loopback - KSZ879x specific function to get loopback
+ *                        configuration status for a specific port
+ * @dev: Pointer to the device structure
+ * @port: Port number to query
+ * @val: Pointer to store the result
+ *
+ * This function reads the SMI registers to determine whether loopback mode
+ * is enabled for a specific port.
+ *
+ * Return: 0 on success, error code on failure.
+ */
+static int ksz879x_get_loopback(struct ksz_device *dev, u16 port,
+				u16 *val)
+{
+	u8 stat3;
+	int ret;
+
+	ret = ksz_pread8(dev, port, REG_PORT_STATUS_3, &stat3);
+	if (ret)
+		return ret;
+
+	if (stat3 & PORT_PHY_LOOPBACK)
+		*val |= BMCR_LOOPBACK;
+
+	return 0;
+}
+
+/**
+ * ksz879x_set_loopback - KSZ879x specific function  to set loopback mode for
+ *			  a specific port
+ * @dev: Pointer to the device structure.
+ * @port: Port number to modify.
+ * @val: Value indicating whether to enable or disable loopback mode.
+ *
+ * This function translates loopback bit of the BMCR register into the
+ * corresponding hardware register bit value and writes it to the SMI interface.
+ *
+ * Return: 0 on success, error code on failure.
+ */
+static int ksz879x_set_loopback(struct ksz_device *dev, u16 port, u16 val)
+{
+	u8 stat3 = 0;
+
+	if (val & BMCR_LOOPBACK)
+		stat3 |= PORT_PHY_LOOPBACK;
+
+	return ksz_prmw8(dev, port, REG_PORT_STATUS_3, PORT_PHY_LOOPBACK,
+			 stat3);
+}
+
 /**
  * ksz8_r_phy_ctrl - Translates and reads from the SMI interface to a MIIM PHY
  *		     Control register (Reg. 31).
@@ -731,16 +782,20 @@ static int ksz8_r_phy_bmcr(struct ksz_device *dev, u16 port, u16 *val)
 	if (ret)
 		return ret;
 
-	if (restart & PORT_PHY_LOOPBACK)
-		*val |= BMCR_LOOPBACK;
-
 	if (ctrl & PORT_FORCE_100_MBIT)
 		*val |= BMCR_SPEED100;
 
 	if (ksz_is_ksz88x3(dev)) {
+		if (restart & KSZ8873_PORT_PHY_LOOPBACK)
+			*val |= BMCR_LOOPBACK;
+
 		if ((ctrl & PORT_AUTO_NEG_ENABLE))
 			*val |= BMCR_ANENABLE;
 	} else {
+		ret = ksz879x_get_loopback(dev, port, val);
+		if (ret)
+			return ret;
+
 		if (!(ctrl & PORT_AUTO_NEG_DISABLE))
 			*val |= BMCR_ANENABLE;
 	}
@@ -1001,8 +1056,7 @@ static int ksz8_w_phy_bmcr(struct ksz_device *dev, u16 port, u16 val)
 
 	restart = 0;
 	restart_mask = PORT_LED_OFF | PORT_TX_DISABLE | PORT_AUTO_NEG_RESTART |
-		PORT_POWER_DOWN | PORT_AUTO_MDIX_DISABLE | PORT_FORCE_MDIX |
-		PORT_PHY_LOOPBACK;
+		PORT_POWER_DOWN | PORT_AUTO_MDIX_DISABLE | PORT_FORCE_MDIX;
 
 	if (val & KSZ886X_BMCR_DISABLE_LED)
 		restart |= PORT_LED_OFF;
@@ -1022,8 +1076,16 @@ static int ksz8_w_phy_bmcr(struct ksz_device *dev, u16 port, u16 val)
 	if (val & KSZ886X_BMCR_FORCE_MDI)
 		restart |= PORT_FORCE_MDIX;
 
+	if (ksz_is_ksz88x3(dev)) {
+		restart_mask |= KSZ8873_PORT_PHY_LOOPBACK;
+
 		if (val & BMCR_LOOPBACK)
-		restart |= PORT_PHY_LOOPBACK;
+			restart |= KSZ8873_PORT_PHY_LOOPBACK;
+	} else {
+		ret = ksz879x_set_loopback(dev, port, val);
+		if (ret)
+			return ret;
+	}
 
 	return ksz_prmw8(dev, port, regs[P_NEG_RESTART_CTRL], restart_mask,
 			 restart);

drivers/net/dsa/microchip/ksz8795_reg.h

@@ -265,6 +265,7 @@
 #define PORT_AUTO_MDIX_DISABLE		BIT(2)
 #define PORT_FORCE_MDIX			BIT(1)
 #define PORT_MAC_LOOPBACK		BIT(0)
+#define KSZ8873_PORT_PHY_LOOPBACK	BIT(0)
 
 #define REG_PORT_1_STATUS_2		0x1E
 #define REG_PORT_2_STATUS_2		0x2E